1. Field of the Invention
The invention concerns a semiconductor power module with an insulating fluid (e.g., liquid) cooling system.
2. General State of the Art
Semiconductor power modules can be used as inverters or other activation devices. Modules of this type can include a plurality of parallel-connected semiconductor devices such as transistors T, MOSFET or IGBT devices of the type schematically illustrated in FIG. 1 and shown in greater detail in FIGS. 2 and 3. A plurality of such devices are formed on a chip (silicon element) F. A copper slice A carries an insulating thin layer of alumina D which in turn carries a copper galvanic deposit G. One or more chips F are welded on the copper galvanic deposit G. The lower surface of the copper slice A is connected to a radiator or heat sink H cooled by water or air circulation. The upper side of the copper slice A, carrying the insulating alumina layer D and the chips F, is immersed in a silicone resin I for protecting the chips F from environmental damage from air, water, dust, and the like.
As shown schematically in FIG. 1, each chip F has one or more transistors T formed thereon, each transistor T having an emitter E, a base B, and a collector C. Wires are connected to corresponding contacts to allow connection with other transistors in the module.
It is inconvenient to employ several transistors of the type described above for power modules because the materials used, particularly the copper slice A, the insulating alumina layer D, the copper layer G and the silicon elements F, have different coefficients of thermal expansion. As a result, thermal cycling during operation breaks the welds and thus causes the transistors and the whole module to which they are secured to fail. The welded contact between the copper deposit G and external connection terminals are especially prone to such breakage.
Moreover, the insulating alumina layer D has a poor coefficient of heat transmission and thus reduces the heat exchange between the chips F and the radiator or heat sink H, thereby reducing the heat dissipation during operation of the module. Above a certain power, this reduction in the heat dissipation leads to a phenomenon called "heat leak," in which the dispersed power increases without suitable heat dissipation. Heat leak is especially a problem with field-effect transistors, whose coefficients of thermal expansion increase with temperature. In addition, the silicone resin is an insulator which forms a thermal barrier, further limiting heat dissipation.